This invention relates to an electric circuit protector that comprises a plurality of liquid-metal type current-limiting devices and, more particularly, relates to a circuit protector of this character which is capable both of carrying high continuous currents and of effectively limiting letthrough currents under fault conditions.
Examples of liquid-metal current-limiting devices are disclosed in U.S. Pat. Nos. 3,117,203-Hurtle; 3,389,359-Harris; and 3,501,730-Ito et al. These devices typically carry continuous current through a column of liquid metal which has a low resistance at normal temperatures. When a fault current flows through the column, a portion of the liquid metal is abruptly vaporized into a high-pressure vapor by the high temperature resulting from the fault current, and this vapor has a high resistivity that limits the fault current. Thereafter, the vaporized metal is allowed to cool and return to its liquid state so that it recovers its original low resistance, thus permitting reuse of the current-limiting device.
Current-limiting devices of the above type are capable of limiting the maximum instantaneous peak value of the current permitted to flow (i.e., the maximum let-through current) to a value considerably lower than the maximum instantaneous peak value which would have been carried by the device had it retained its normal impedance (i.e., the maximum prospective current), as illustrated, for example, in FIG. 9 of the aforesaid Hurtle patent.
Most current-limiting devices, even silver-sand current limiting fuses, are characterized by reduced effectiveness in their current-limiting action and the continuous current rating of the device increases. Accordingly, as a general rule, the higher the continuous current rating of the currentlimiting device, the higher will be the let-through current for a given prospective fault current. Insofar as I am aware, for continuous current ratings of 2,000 or 3,000 amperes, no effective current-limiting protection is presently available.
In a current-limiting device that comprises a conducting element that changes in phase in response to fault currents, current-limiting effectiveness can be increased by providing the conductive element with a zone of restricted cross-section having a relatively high resistance and a relatively low mass (and hence a relatively low thermal capacity). This high resistance and low thermal capacity accelerate melting and/or vaporization of the restricted cross-section portion in response to a fault-current initiation, thereby accelerating current-limiting action. But there high resistance can produce overheating under high continuous current conditions, thus interfering with the ability of the conductive elements to carry high continuous currents. Higher continuous current-carrying ability can be obtained by effectively cooling the restricted zone of the conductive element under continuous current conditions.